Cells from renewable tissues can permanently withdraw from the cell cycle in response to diverse stress, including dysfunctional telomeres, DNA damage, strong mitogens and disrupted chromatin. This response, termed cellular senescence, is controlled by the p53 and RB tumor suppressor proteins and constitutes a potent mechanism for preventing the cancer. Nonetheless, mounting evidence suggest that, in complex organisms such as mammals, senescent cells can also contribute aging. Thus, the senescence response may be an example of evolutionary antagonistic pleiotropy. Senescent cells acquire a complex phenotype. In addition to the essentially irreversible arrest of cell proliferation, many senescent cells become resistant to apoptotic cell death, which may explain why senescent cells accumulate with age. In addition, senescent cells show prominent changes in function, a key feature of which, at least in stromal fibroblasts, is the secretion of biologically active molecules that can disrupt normal tissue microenvironments and affect the behavior of neighboring cells. We term this feature of senescent cells the senescent secretory phenotype. Very little is known about how the senescence secretory phenotype is controlled. We propose to fill this gap in our knowledge. Moreover, the idea that the senescence response is antagonistically pleiotropic is a working hypothesis that rests largely on studies of fibroblasts in relatively simple cell culture systems. We therefore propose to characterize the senescent phenotype more broadly, using more sophisticated cell systems, cell types other than fibroblasts, and functional cell-based assays. We propose to define the senescent states of human fibroblasts using biochemical, cytological and functional assays, determine whether different stimuli are functionally equivalent, and how stimuli implicated in organismal aging affect the senescent phenotype of cells. We propose to compare senescent states between human of different origins, and between human and mouse cells in order to more effectively model human cell senescence in the mouse. Finally, we will determine the roles of the p53 and pRB tumor suppressors in establishing and maintaining the functional status and senescent secretory phenotypes of human and mouse cells, and explore strategies for altering the function of senescent cells. These experiments will provide crucial insights into how senescent human cells, and indirectly the p53 and pRB tumor suppressors, may contribute to aging, how we might better model this process in the mouse, and how we might modify the senescent phenotype to ameliorate their deleterious effects.